Mac Renovations has been using the home's interior square feet for his exterior paint cost estimations. It was brought to his attention that adding a home's external openings might also be useful in predicting painting cost estimations.

The following data was gathered from the last $12$ houses he had painted.

Compute the total cost equation for the currently used interior square feet independent variable

Compute the total cost equation when both interior square feet and external openings are used as independent variables Which total cost equation is more reliable and why?

$($check figure: linear regression fixed costs $\backslash (=\backslash$$ $789\mathrm{.}62\backslash ))$

$($check figure: multiple regression fixed costs $\backslash (=\backslash$$ $1,385\mathrm{.}74\backslash ))$